Injection moulding of PVC, both rigid [PVCu] and flexible [PVCp] types is relatively straight forward but care needs to taken to achieve optimum results. PVC is an extremely versatile material, in its plasticised form [PVCp] it is available in many levels of softness, suitable for applications from footwear to seals, gaiters and covers. Rigid PVC [PVCu], in contrast, is a hard glossy material with good exterior performance, a degree of inherent flame retardancy and can be readily tailored for a variety of impact strengths and applications. Too often overlooked because of perceived difficulties in processing they are often substituted with less suitable materials for the required application but with more forgiving processing characteristics.
PVC is extremely sensitive to excessive heat, either direct or shear heating. Ideal processing temperatures are between 170 - 190 C and residence times for material in the barrel should be kept to a minimum. Failure to observe these basic criteria will result in thermal degradation of the PVC, either as visible discolouration or, in extreme cases, breaking down of the polymer with large quantities of an unpleasant, corrosive gas [HCl] being produced, which is to be avoided at all cost. Extended stoppage periods without emptying the barrel of polymer, purging if necessary, should also be avoided. Proprietary purging and/or freezing compounds are recommended.
Generally any modern screw injection moulding moulder equipped with a GP or PVC screw tip is suitable for moulding injection grades of PVC. The nozzle needs to be as short as possible and should have good temperature control [not just a "simmerstat"]. The size of the machine needs to selected with regard to the ratio of barrel capacity to shot size so as to ensure residence time in the barrel is kept to a minimum.
Good control of the machine parameters is essential, temperature, pressure, rotational speed all need to be regulated accurately. The melt temperature of the PVC is carefully controlled by a combination of set temperatures, screw speed and back pressure. Typical set temperatures are:
The heaters contribute to the melt temperature as does the frictional shear heat producing a typical melt temperature of 200 - 205C. This should produce a fully homogenised polymer with a uniform melt and a gelation point in the mixing area of the screw.
Rigid grades mould better with slower screw speeds to avoid excessive shear. Flexible grades can be moulded with much faster screw rates.
Injection speed and pressure should be the minimum required to ensure a slow, smooth filling of the cavity. Clamp pressure to hold the cooling moulding in the tool and setting the shot size to ensure adequate filling of the tool and the maximum amount of the PVC is discharged from the area in front of the screw [leaving the smallest amount possible to be part of the next shot] should also be established.
Although stainless steel tools are preferred this is not essential although steel will need protecting from the aggressive, corrosive effects of degradation gasses. Hot runner tools are not recommended for PVC Hard chromium plating of the cavity is also preferred. regular use of corrosion inhibiting sprays is also recommended.
Typical shrinkage values for PVC on cooling are:
The sprue length and runners should be as short as possible, ideally with a maximum flow path to thickness ratio of:
Gate size [the point where the PVC enters the moulding cavity] is critical as this can generate frictional heat, whic can possibly burn or degrade the polymer. A pin gate for PVCp is typically 1mm diameter but PVCu would require a minimum of 2mm or larger for small mouldings and upto 20mm for large mouldings. Other gate types might also be considered that would not restrict the flow of the molten polymer as much.
Design of the moulding requires careful consideration. Sharp corners or pockets should be avoided to minimise the possibility of localised burning. Where a radius is required it needs to be a minimum of 0.8mm on the internal corner but external corners can be sharp. Failure to observe this radius requirement could cause stress issues in the moulded component.
Where ribs are included, to brace the rigidity of a component, they need to be tapered. The thickness of the rib base should be no more than 75% of the wall section and taper one degree each side of the rib, with a radius at the base of the rib of half the rib thickness.
Good venting will also reduce the likelihood of material decomposition [loose fitting ejector pins can also assist as extra venting and help in preventing this].
Following these general guidelines should result in successful mouldings. In house colouring with Vynacol masterbatch is no more complex than colouring any other polymer.